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Surface melting on small particles was detected using x-ray diffraction from Pb crystallites of 50 nm diam in ultrahigh vacuum. Since the melting temperature depends on particle size, it can be difficult to distinguish between the melting and surface melting. A sharp crystallite size distribution is obtained by melting the smaller particles while a liquid skin forms on the remaining crystallites. When the sample is cooled slightly, the liquid particles remain liquid while the liquid skin on the crystallites solidifies. In situ x-ray diffraction reveals that the liquid skin thickness changes reversibly.

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